The present invention relates to a receiver for digital audio broadcasting as described in Recommendation BS.774 of the Radiotelecommunication Sector of the International Telecommunications Union (ITU-R), entitled xe2x80x9cService requirements for digital sound broadcasting to vehicular, portable, and fixed receivers using terrestrial transmitters in the VHF/UHF bands,xe2x80x9d more particularly to the method by which the receiver detects the center frequency of the digital audio broadcast signal.
Digital audio broadcasting, also referred to as DAB, employs orthogonal frequency division multiplexing (OFDM) together with powerful error-correcting functions to transmit digital data at high speed with high reliability, even to mobile receivers which may be strongly affected by multipath fading. The digital data are modulated onto a large number of closely spaced OFDM carrier signals, which are combined into a single broadcast signal. A DAB receiver includes an automatic frequency control (AFC) circuit that keeps the receiver tuned to the center frequency of the broadcast signal. The AFC function is normally implemented in two stages: a coarse tuning stage that detects frequency error as an integer multiple of the OFDM carrier spacing; and a fine tuning stage that detects frequency errors less than the OFDM carrier spacing.
A conventional coarse tuning method, described in European Patent Application No. 0 529 421 A2, exploits a known constant-amplitude zero-autocorrelation data series, referred to as a CAZAC data series, which is inserted periodically into the broadcast signal as a phase reference symbol. The conventional method is a sliding correlation method that correlates the received phase-reference data series with the known CAZAC series, shifting the two series in relation to each other on the frequency axis until a clear peak correlation is found. Each shift of one data position corresponds to a frequency error equal to the OFDM carrier spacing.
FIG. 1 shows simulated results of the sliding correlation process. The correlated data series contained one hundred twenty-eight values. The values from zero to two hundred on the horizontal axis correspond to frequency errors of up to plus or minus one hundred times the OFDM carrier spacing. The values on the vertical axis indicate the magnitude of the correlation for each assumed frequency error. The large peak in the center represents the correct or true tuning point, with zero frequency error.
FIG. 1 also illustrates a problem in the conventional coarse tuning method: smaller peaks (false peaks) occur at intervals of sixteen OFDM carriers. This is due to the structure of the CAZAC data series, which will be described later. If the clear peak correlation found by the conventional tuning method is one of the false peaks, coarse tuning fails and the broadcast cannot be received.
The false peaks in FIG. 1 cannot be screened out simply from their heights. Under realistic operating conditions, with an imperfect transmission channel, the heights of the peaks vary, even when the total signal strength is constant. Although the true tuning point will always have the highest peak, it is not possible to predict in advance what the height of any of the peaks will be.
Nor is it possible to guarantee that the first peak encountered in a sliding correlation search will be the true peak, so the conventional coarse-tuning method is reliable only if the search range is enlarged to include a fairly large number of peaks, the highest of which can be safely assumed to be the true peak. Conducting a sliding correlation search over such a large range is computationally demanding, however, and requires an expensive, high-speed processor.
It is accordingly an object of the present invention to avoid mistaking false correlation peaks for the true tuning point.
Another object of the invention is to identify false peaks observed in isolation, without having to examine a plurality of peaks.
Another object is to avoid sporadic audio dropouts caused by automatic frequency control errors.
A further object is to enable a digital audio broadcast receiver to employ a comparatively inexpensive processor.
The invented method of tuning a digital audio broadcast receiver employs complex-valued data obtained from frequency-differential demodulation of a phase reference symbol, and comprises the following steps:
(a) sliding correlation of the complex-valued data with a lower series of CAZAC data transmitted on negatively numbered carriers in the phase reference symbol;
(b) sliding correlation of the complex-valued data with an upper series of CAZAC data transmitted on positively numbered carriers in the phase reference symbol;
(c) finding a shift at which the lower correlation coefficient and upper correlation coefficient have a sum with maximum magnitude;
(d) storing the lower correlation coefficient and upper correlation coefficient obtained at this shift;
(e) determining whether these upper and lower correlation coefficients differ in relative magnitude by more than a first amount;
(f) determining whether these upper and lower correlation coefficients differ in phase angle by more than a second amount;
(g) deciding, from steps (e) and (f), whether the shift found in step (c) is valid; and
(h) adjusting the tuning of the digital audio broadcast receiver according to the shift found in step (c), provided the shift is valid.
The invention also provides digital audio broadcast receivers employing the invented method of tuning.